Avoid passing nullptr to sycl::queue::prefetch to avoid buggy hang

[oleksandr-pavlyk]

For bookkeeping filed an issue behind the hang as XDEPS-2765.

Fix for the hang is to avoid passing nullptr argument to sycl::queue::prefetch.

[oleksandr-pavlyk]

Very likely this hang was the reason why the Generate coverage data workflow never got to building the Python extensions.

[oleksandr-pavlyk]

Very likely this hang was the reason why the Generate coverage data workflow never got to building the Python extensions.

No the failure is due to a problem with genhtml:

2021-09-30T18:48:05.7792522Z genhtml: ERROR: cannot read /tmp/test_service-c3cfab.cpp
2021-09-30T18:48:05.7793677Z make[3]: *** [tests/CMakeFiles/lcov-genhtml.dir/build.make:72: tests/CMakeFiles/lcov-genhtml] Error 2
2021-09-30T18:48:05.7794817Z make[2]: *** [CMakeFiles/Makefile2:207: tests/CMakeFiles/lcov-genhtml.dir/all] Error 2
2021-09-30T18:48:05.7795883Z make[1]: *** [CMakeFiles/Makefile2:214: tests/CMakeFiles/lcov-genhtml.dir/rule] Error 2
2021-09-30T18:48:05.7796749Z make: *** [Makefile:221: lcov-genhtml] Error 2

On the next line of the workflow, we are executing import of dpctl, which fails

2021-09-30T18:48:05.7817325Z ModuleNotFoundError: No module named 'dpctl._sycl_context'

but this goes unnoticed, and the worflow script proceeds with running pytest.